The fabrication of many photonic devices has been achieved through exposure of transmissive and absorbing materials to intense laser radiation in order to change the optical properties of said materials. For example, UV-induced photosensitivity of germanium doped silica glasses has been exploited in order to create permanent refractive index changes in the photosensitive Ge-doped silica cores of single mode optical fibers and waveguides as opposed to the undoped cladding. By creating a spatial intensity modulation of the UV exposure either by using a two-beam interference technique as disclosed in U.S. Pat. No. 4,807,950 by Glenn et al. or by using a phase mask as disclosed in U.S. Pat. No. 5,367,588 by Hill et al., Bragg grating structures can be produced in the photosensitive core of the waveguide.
As disclosed by Glenn et al., permanent periodic gratings are provided or impressed into the core of an optical fiber by exposing the core through the cladding to the interference fringe pattern generated by two coherent beams of ultraviolet laser light that are directed against the optical fiber symmetrically to a plane normal to the fiber axis. The material in the fiber core is exposed to the resultant interference fringe intensity pattern created by the two overlapping UV beams creating permanent periodic variations in the refractive index along the length of the UV photosensitive core of the waveguide. The resultant index variations are oriented normal to the waveguide axis so as to form the Bragg grating.
A more popular method of photo imprinting Bragg gratings is taught by Hill et al. in U.S. Pat. No. 5,367,588 where an interference fringe pattern is generated by impinging a single UV light beam onto a transmissive diffractive optic known as a phase mask. The waveguide to be processed is placed immediately behind the phase mask and is exposed to the generated interference fringe pattern leading to the formation of the Bragg grating structure. In these prior art examples, optical fibers or waveguides having a Ge doped photosensitive core are irradiated with UV light at a predetermined intensity and for a predetermined duration of time sufficient to obtain a substantially permanent Bragg grating structure within the core of said waveguide.
These prior art gratings provide a useful function, however they suffer from some limitations in terms of the amount of induced index change that is possible. In order for some Bragg grating structures to be written in a standard telecommunications single mode optical fiber, the optical fiber often needs to be photosensitized to UV light by exposing such an optical fiber to hydrogen or deuterium gas at elevated pressures and temperatures as taught by Atkins et al. in U.S. Pat. No. 5,287,427 or by hydrogen flame brushing as taught be Bilodeau et al. in U.S. Pat. No. 5,495,548. After exposure, the UV written structures need to be annealed at elevated temperatures in order to remove any remaining interstitial hydrogen or deuterium present in the waveguide core. As taught by Erdogan et al. in U.S. Pat. No. 5,620,496, this annealing step is often implemented in order to stabilize by accelerated aging, the induced index change. These extra processing steps to the optical fiber or waveguide complicate the manufacturing of photonic devices and reduce yield.
Another method for creating permanent photoretractive index changes in glasses employs the use of intense UV beams with fluences or energy/unit-area per laser pulse densities that approach those required to produce macroscopic damage of the glass. Askins et al. in U.S. Pat. No. 5,400,422 teach a method for producing permanent photoretractive index changes in the photosensitive cores of Ge-doped optical fibers with single high intensity UV laser pulses. The high intensity portions of the interference fringes created by two crossed UV beams split from a single UV beam create localized damage at the core-cladding interface within the fiber. Because the process for inducing index change is one of structural change due to localized physical damage to the glass, rather than due to UV photoinduced color center formation, the induced index change is more robust and does not decrease with elevated temperature. Thus, annealing steps as taught by Erdogan et al. in U.S. Pat. No. 5,620,496 are not required. In fact Askins et al. disclose that gratings produced in this way cannot be removed by annealing until the fiber or waveguide approaches the material's glass transition temperature. The drawback of this approach for induction of index change is that the Bragg gratings produced in this fashion have relatively low refractive index modulations (Δn=10−5) and are mechanically weak since the effective refractive index change results from periodic localized damage at the core-cladding interface. Since the damage mechanism is based on an intensity threshold process, the spectral quality of the resulting Bragg grating is often poor.
Recently processes that employ high-intensity laser pulses in the femtosecond pulse duration regime for creating permanent changes in the refractive indices of glasses have been explored by several groups of researchers. K. M. Davis et al. disclose a technique for inducing index change in bulk glasses with ultra-high peak power femtosecond infra-red radiation in Opt. Lett 21, 1729 (1996). The creation of waveguides in bulk glasses using this technique is taught by Miura et al. in U.S. Pat. No. 5,978,538 while the modification or trimming of existing waveguide structures is taught by Dugan et al. in U.S. patent application No. 20030035640. The physical process that appears to cause the refractive index change in the materials is due to the creation of free electrons through non-linear absorption and multi-photon ionization of bound charges, followed by avalanche ionization and localized dielectric breakdown as these free electrons are accelerated by the intense but short time duration laser field. Also, this leads to a localized melting and restructuring of the material and a concurrent increase in the index of refraction. Work performed in this field has used laser pulses that are tightly focused to near-diffraction limited spot sizes generating extremely high intensities of light, approximately 1014 W/cm2, in order to initiate non-linear absorption processes in the materials. While this allows for high-resolution spatial localization of the refractive index change, it involves point-by-point scanning of the ultra-short-time-duration laser along the length of the optical fiber or waveguide as disclosed by Fertein et al. Appl. Opt. 40 (21), 3506 (2001). This is a great disadvantage for writing retroreflective Bragg grating structures but is suitable for writing long-period Bragg grating structures which, instead of coupling light from the forward-propagating guided mode into a retro-reflecting guided mode, couple light energy traveling along the fiber in a forward-propagating guided mode into light that propagates into forward-propagating cladding modes where the light is at least partially attenuated. There are several prior-art examples of long-period grating fabrication. The point-by-point writing method is taught by Hill et al. in U.S. Pat. No. 5,104,209 using a slit-amplitude mask. A variation on the amplitude mask technique is taught by Tam in U.S. Pat. No. 6,208,787 where a plano-convex array of cylindrical microlenses is used to focus portions of an incident UV beam onto on optical fiber. Another technique for fabrication of long-period fiber gratings with an electric arc is taught by Kosinski et al. in U.S. Pat. No. 6,050,109.
In order to photo imprint retroreflective Bragg structures into the core of optical fibers or waveguides using high-intensity femtosecond time duration radiation, it is advantageous to generate an interference fringe pattern originating from a single femtosecond laser pulse either using a holographic technique or a diffractive optic. Kawamura et al. in Appl. Phys. Lett. 78 (8), 1038 (2001) disclose an apparatus for producing a hologram using a two-beam laser interference exposure process, comprising the steps of using a femtosecond laser having a pulse width of 10 to 900 femtoseconds and a peak output of 1 GW or more that is capable of generating a pulse beam at or close to the Fourier transform limit. The beam from the laser is divided into two beams using a beam splitter, controlled temporally through an optical delay circuit and spatially using plane and concave mirrors each having a slightly rotatable reflection surface to converge the beams on a surface of or within a substrate for recording a hologram at an energy density of 100 GW/cm2 or more with keeping each polarization plane of the two beams in parallel so as to match the converged spot of the two beams temporally and spatially, whereby a hologram is recorded irreversibly on the substrate formed of a transparent material, semiconductor material or metallic material. The volume hologram is optionally layered so as to provide a multiplex hologram recording that is permanent unless it is heated to a temperature to cause the structural change in the atomic arrangement of the substrate in which the hologram is inscribed. The authors teach this method in US Pat. Appl.20020126333.
Maznev et al. Opt. Lett. 23 (17), 1378 (1998) disclose a technique for generating interference fringes with femtosecond pulses by overlapping two femtosecond beams that originate from a single beam which passes through a diffractive optical element. This method is taught by the authors in U.S. Pat. No. 6,204,926. Miller et al., in U.S. Pat. No. 6,297,894, teach a similar method for utilizing a diffractive optic to generate an interference fringe pattern in order to induce refractive index changes in materials using femtosecond time duration laser radiation. An exemplary embodiment of the invention of Miller et al. comprises a femtosecond laser source for providing light to a diffractive optical element. Light propagating from the diffractive optical element is incident on a curved mirror, which acts to focus the light into a lens or another curved mirror and then into a target.
It is an objective of this invention to overcome the aforementioned limitations within the prior art systems of inducing refractive index change in optical fibers and waveguides using femtosecond time duration laser radiation. Additionally, it would be beneficial to provide a simple method of producing high quality FBGs that are robust and are not subject to annealing.